1. Field of the Invention
The present invention relates to a charge coupled device (CCD) or in particular to a charge detection device therein and a driver thereof.
2. Description of the Prior Art
An floating diffusion amplifier (FDA) and a floating gate amplifier (FGA) are known as representatives of charge detection devices of a charge coupled device (CCD). Especially a floating gate amplifier can realize an amplifier without reset noises and without destruction. Details of a floating gate amplifier is described in following papers: Wen and Salsbury, "Analysis and Design of a Single Stage floating Gate Amplifier", ISSCC Dig. Tech. Papers, February 1973 pp. 154-155; and Wen, "Design and Operation of a floating Gate Amplifier", IEEE Journal of Solid-State Circuits, Vol. SC-9, No.6, December 1974, pp. 410-414.
However, a prior art charge detection device with a floating gate amplifier has a problem that the sensitivity of charge detection is low. This is ascribed to a parasitic capacitance. Because a floating gate has to be extended to a first stage transistor of a source follower, a parasitic capacitance cannot be reduced to a large extent. The above papers state that it is important to increase capacitance C1 and decrease capacitances C2, C3, C4 and Cin.
FIG. 1A shows a plan view of a charge detection device of a charge coupled device around a floating gate amplifier thereof and FIG. 1B shows a section along a line A--A' in FIG. 1A. This charge coupled device corresponds to prior art Wen's device. A P well 97 and an n layer 98 and a gate oxide film 96 are layered successively, and a floating gate 95 is arranged on the gate oxide film 96 similarly to gates .o slashed.1-.o slashed.3. Further, a bias gate 94 is formed above the floating gate 95. The floating gate 95 overlaps with gates .o slashed.1 and .o slashed.3. In an actual floating gate amplifier, parasitic capacitances other than the above-mentioned ones also exist. A parasitic capacitance C6 exists between the floating gate 95 and the gate .o slashed.1, while another parasitic capacitance C7 exists between the floating gate 95 and the gate .o slashed.3. Then, the sensitivity of the floating gate amplifier is as low as 0.4-0.5 times that of a floating diffusion amplifier.
Further, because the floating gate 95 is floating electrically, when a leakage current exists, a potential of the floating gate fluctuates, and this affects an output signal. At present, any insulating film cannot reduce the leakage current to zero. There is a tendency that the leakage current increases with increasing potential of the floating gate 95. Further, in FIG. 1B, an etching boundary of the bias gate 94 of aluminum is formed above the n layer 98 (a channel of signal charge Q). Then, it is observed that a leakage current increased between the floating gate 95 and the bias gate 94 and between the floating gate 95 and the n layer 98 because a stress is applied to around the etching boundary in a heating process.